1. Field of the Invention
The invention relates to the preparation of synthetic lubricant base stocks, and more particularly to synthetic lubricant base stocks made by dimerizing long-chain linear olefins.
2. Description of Related Methods
Synthetic lubricants are prepared from man-made base stocks having uniform molecular structures and, therefore, well-defined properties that can be tailored to specific applications. Mineral oil base stocks, on the other hand, are prepared from crude oil and consist of complex mixtures of naturally occurring hydrocarbons. The higher degree of uniformity found in synthetic lubricants generally results in superior performance properties. For example, synthetic lubricants are characterized by excellent thermal stability. As automobile engines are reduced in size to save weight and fuel, they run at higher temperatures, therefore requiring a more thermally stable oil. Because lubricants made from synthetic base stocks have such properties as excellent oxidative/thermal stability, very low volatility, and good viscosity indices over a wide range of temperatures, they offer better lubrication and permit longer drain intervals, with less oil vaporization loss between oil changes.
Generally, synthetic base stocks are prepared by oligomerizing internal and alpha-olefin monomers to form a mixture of dimers, trimers, tetramers, and pentamers, with minimal amounts of higher oligomers. The unsaturated oligomer products are then hydrogenated to improve their oxidative stability. The resulting synthetic base stocks have uniform isoparaffinic hydrocarbon structures similar to high quality paraffinic mineral base stocks, but have the superior properties mentioned due to their higher degree of uniformity.
Synthetic base stocks are produced in a broad range of viscosity grades. It is common practice to classify the base stocks by their viscosities, measured in centistokes (cSt) at 100.degree. C. Those base stocks with viscosities less than or equal to about 4 cSt are commonly referred to as "low viscosity" base stocks, whereas base stocks having a viscosity in the range of around 40 to 100 cSt are commonly referred to as "high viscosity" base stocks. Base stocks having a viscosity of about 4 to about 8 cSt are referred to as "medium viscosity" base stocks. The low viscosity base stocks generally are recommended for low temperature applications. Higher temperature applications, such as motor oils, automatic transmission fluids, turbine lubricants, and other industrial lubricants, generally require higher viscosities, such as those provided by medium viscosity base stocks (i.e. 4 to 8 cSt grades). High viscosity base stocks are used in gear oils and as blending stocks.
The viscosity of the base stocks is determined by the length of the oligomer molecules formed during the oligomerization reaction. The degree of oligomerization is affected by the catalyst and reaction conditions employed during the oligomerization reaction. The length of the carbon chain of the monomer starting material also has a direct influence on the properties of the oligomer products. Fluids prepared from shortchain monomers tend to have low pour points and moderately low viscosity indices, whereas fluids prepared from long-chain monomers tend to have moderately low pour points and higher viscosity indices. Oligomers prepared from long-chain monomers generally are more suitable than those prepared from shorter-chain monomers for use as medium viscosity synthetic lubricant base stocks.
One known approach to oligomerizing long-chain olefins to prepare synthetic lubricant base stocks is to contact the olefin with boron trifluoride together with a promotor at a reaction temperature sufficient to effect oligomerization of the olefin. See, for example, co-assigned U.S. Pat. Nos. 4,400,565; 4,420,646; 4,420,647; and 4,434,308. However, boron trifluoride gas (BF.sub.3) is a pulmonary irritant, and breathing the gas or fumes formed by hydration of the gas with atmospheric moisture poses hazards preferably avoided. Additionally, for some applications, such as semi-synthetic oils or where low temperature properties are important, a higher dimer to trimer ratio than that obtained using such conventional oligomerization catalysts is desirable.
A method for dimerizing long-chain olefins using a less hazardous catalyst is taught in co-assigned U. S. Pat. No. 4,367,352 to Watts, Jr. et al., which discloses the use of a perfluorosulfonic acid resin to dimerize long-chain alpha-olefins. At column 3, the '352 patent teaches that the perfluorosulfonic acid resin produces a high dimer to trimer ratio, and gives an example showing percent dimer and percent trimer in a ratio of about 4.77:1. Applicants have discovered, surprisingly, that a substantially higher dimer/trimer ratio may be obtained by contacting the olefin feed with a catalyst comprising a silica gel alkylsulfonic acid. Like the resins of the '352 Patent, the silica gel alkylsulfonic acids also are less hazardous and more easily handled than boron triflouride. Applicants believe it was heretofore unknown in the art to use silica gel alkylsulfonic acids to prepare synthetic lubricant base stocks having a very high percentage of dimers. By maintaining a low percentage of trimer and higher oligomers in the reaction product, Applicants are able to obtain base stocks having excellent low temperature properties while using long-chain monomers as feedstock.